The U.S. State Department says it cannot find 15 emails from Hillary Clinton during her time as the top U.S. diplomat.

The agency said Thursday it discovered that former Secretary of State Clinton's email records were incomplete after learning that longtime Clinton friend and confidante Sidney Blumenthal handed over several previously undisclosed emails to U.S. lawmakers investigating the deadly 2012 attack on the diplomatic staff in Benghazi, Libya.

"She has turned over 55,000 pages of materials to the State Department," Nick Merrill, a Clinton campaign spokesman said, "including all emails in her possession from Mr. Blumenthal."

Clinton, who is widely expected to be the Democratic party's nominee for the 2016 presidential race, revealed earlier this year that she kept a private email account and server during her State Department tenure. The private address was connected to a non-governmental server kept at her house.

Trey Gowdy, the Republican congressman in charge of the congressional committee investigating the attack on Benghazi, said Clinton's email record "raises serious questions… This has implications far beyond Libya, Benghazi, and our committee's work. This conclusively shows her email arrangement with herself, which was then vetted by her own lawyers has resulted in an incomplete record."

Clinton says she gave all her State Department-related emails to the agency and then destroyed other emails that she said were personal.

U.S. Ambassador Chris Stevens was among four Americans killed in the Benghazi attack.